The present invention relates to a scanning pattern drawing apparatus such as a laser plotter that draws patterns by scanning with laser light. In particular, the present invention relates to a mechanism for detecting both the distance between the drawing surface and the associated optical system and the inclination of the drawing surface with respect to the latter and which corrects the detected data if necessary.
Conventional laser plotters are generally intended to draw patterns with a comparatively low precision. The diameter of the beam spot which determines the minimum line width is typically on the order of 30 .mu.m. To allow for pattern drawing with a higher precision and a smaller line width, the spot diameter must be further reduced by shortening the focal length of the scanning lens. However, a scanning lens with a shorter focal length unavoidably has a smaller depth of focus, so that if the workpiece on which a pattern is to be drawn is displaced either upward or downward with respect to the optical system, the entire drawing surface may be located beyond the depth of focus. On the other hand, if the drawing surface is inclined with respect to the optical system, part of a scanning field or subfield may be located beyond the depth of focus.
To overcome these problems, some provision must be made for detecting the displacement or inclination of the workpiece and performing the necessary corrections. In practice, however, a large scanning pattern drawing apparatus involves considerable difficulty in focusing at various positions of a single scan stroke by vertical movements of the drawing surface, and hence it is desired that the position of the drawing surface be held constant for a single stroke of scanning.
If detection of the position of the drawing surface relative to the optical system is performed at only one point, failure to focus is most likely to occur. Consider, for example, the case of performing position detection at one point on the center line of the scan width. This presents no problem if the drawing surface is level over the entire portion of the scan width or if it is inclined linearly. However, if its inclination is not linear, as shown in FIG. 37, and if the drawing surface is driven vertically on the basis of the results of detection at a central point B, the focus will be at the dashed line. In other words, only one limit of the depth of focus of the lens is utilized, and point A may be located beyond this depth of focus. If, on the other hand, the drawing surface is driven vertically on the basis of the average of the results of detection at two points A and C, the focus will be at the dot-dash line. Thus, both limits of the depth of focus are effectively used to increase the possibility that all points on the drawing surface will fall within the proper range of focus.
However, if the profile of the drawing surface is curved upward as shown in FIG. 38, vertically moving the drawing surface on the basis of the results of detection at the two end points A and C is not recommendable since point B is highly likely to be located beyond the depth of focus. In such a case, the results of detection at three points A, B and C are desirably averaged to provide a basis for vertical movements of the drawing surface.
Thus, the possibility of drawing patterns with high precision can be increased by increasing the number of detection points even if focusing is to be achieved merely by moving the drawing surface in a vertical direction.